"I vowed that if my initiative did not succeed, I would resign
and I have done so," Mr Jebali told a news conference after meeting
President Moncef Marzouki.

Describing his step as "a big disappointment", he said he was standing down to "fulfil a promise made to the people."
"Our people are disillusioned by the political class. We must restore confidence," he stressed.

And he added: "The failure of my initiative does not mean the
failure of Tunisia or the failure of the revolution," in a reference to
the popular unrest two years ago that ousted autocratic leader Zine
al-Abidine Ben Ali.

Mr Jebali's resignation comes despite comments by Ennahda's
leader Rached Ghannouchi on Monday that all parties involved in the
coalition building talks had wanted the prime minister to remain in
office.

The Ennahda has also indicated that it still hopes it can pull
together a coalition to lead the country into early elections, the BBC's
diplomatic correspondent Bridget Kendall reports.

Mr Ghannouchi is expected to meet President Moncef Marzouki on Wednesday to discuss the growing crisis, reports say.

But in the meantime, our correspondent says, the worry is that
the continuing political stand-off and paralysed economy could lead to
new mass protests and more violence.

On Tuesday, credit rating agency Standard and Poor's said it had
downgraded Tunisia's rating, citing "a risk that the political situation
could deteriorate further amid a worsening fiscal, external and
economic outlook".

Supporters of the secular opposition have blamed Ennahda for Mr Belaid's assassination - an accusation the party denies.

It was the first political assassination in Tunisia since the
popular unrest in 2011 forced the country's autocratic leader Zine
al-Abidine Ben Ali from power and also inspired the so-called "Arab
Spring" uprisings across the region.